Single-atomically dispersed metal / unconventional-phase transition-metal dichalcogenide nanosheet hybrids and methods of preparation and use thereof

ABSTRACT

A single-atomically dispersed metal/two-dimensional transition-metal dichalcogenide nanosheet hybrid comprising a plurality of single-atomically dispersed metal atoms disposed on at least one surface of a transition-metal dichalcogenide nanosheet, wherein the transition-metal dichalcogenide nanosheet is uniformly crystalline.

TECHNICAL FIELD

The present disclosure relates to single-atomically dispersed metal/unconventional-phase transition-metal dichalcogenide nanosheet hybrids useful as for catalyzing various chemical and/or electrical transformations, such as electrochemical hydrogen evolution, CO₂ reduction, nitrogen reduction, nitrite reduction, nitrate reduction, aldehyde oxidation, and the like, and methods of use thereof.

BACKGROUND

Two-dimensional (2D) transition-metal dichalcogenide (TMD) nanosheets (NSs) have attracted increasing interest owing to their unique physicochemical properties and promising applications in energy storage, electronic devices, and electrocatalysis. In particular, the construction of TMD-based hybrid nanostructures has become a promising strategy to further boost their performances in the aforementioned applications. Until now, 2D MoS₂-templated metal nanostructures have been extensively used for electrocatalysis. Although tremendous efforts have been devoted to preparing metal-MoS₂ hybrids, the used MoS₂ templates are in the thermodynamically stable 2H phase or mixed phases. For instance, MoS₂ NSs with mixed 1T and 2H phases, prepared by the electrochemical lithium intercalation and exfoliation, have been used as templates to realize the epitaxial growth of metal nanostructures, e.g., Pt, Pd, and Ag. However, on such mixed-phase MoS₂ templates with numerous phase boundaries and defects, only ˜65% of the synthesized Pt nanoparticles (PtNPs) were epitaxially grown. The lack of methods for preparing 1T- or 1T′-MoS₂ NSs with high phase purity makes it challenging to investigate the effect of the crystal phase of MoS₂ on the templated growth of metals.

Metal/MoS₂ hybrids exhibit great potential towards the electrochemical hydrogen evolution reaction (HER). Normally, the metallic 1T-MoS₂ and semi-metallic 1T′-MoS₂ have much smaller charge transfer resistances compared to the semiconducting 2H-MoS₂, and thus exhibit improved HER performance. As a result, metal/MoS₂ hybrids constructed using MoS₂ templates with mixed phases, i.e., 2H phase mixed with 1T or 1T′ phase, showed better HER activity than those obtained with the pure 2H-MoS₂ templates. However, the mixed phases of the MoS₂ templates severely limit the further improvement of the metal-MoS₂ hybrids towards HER due to the poor conductivity of the 2H-MoS₂. Therefore, constructing metal/MoS₂ hybrids based on 1T- or 1T′-MoS₂ NSs with high phase purity is one of the effective ways to prepare highly efficient HER electrocatalysts.

There is thus a need for improved methods for preparing metal/TMD NS hybrids and products thereof.

SUMMARY

Provided herein is a method of preparing unconventional phase TMD NSs, such as 1T′-MoS₂ NSs, with high phase purity, which are then used as templates to grow single-atomically dispersed metals. Importantly, it is found that single-atomically dispersed metals, such as Au, Ag, Pt, Jr, Ni, Sn, Bi, Cu can be grown on TMD NSs to form the single-atomically dispersed metal/TMD NS hybrids. As a proof-of-concept application, the obtained s-Pt/1T′-MoS₂ is used as an electrocatalyst for HER, which exhibits superior HER performance with a low overpotential of only 10 mV to reach the current density of 10 mA cm⁻², outperforming commercial Pt/C and previously reported Pt-based electrocatalysts. Impressively, the s-Pt/1T′-MoS₂ can achieve high current densities of 1,000, 1,500 and 2,000 mA cm⁻² at overpotentials of ˜91, 112 and 131 mV, respectively, which are much lower than the corresponding overpotentials of the commercial Pt/C, i.e., ˜274, 372 and 450 mV, respectively. To the best of our knowledge, this high-current-density HER performance is the best compared to the previously reported electrocatalysts. More importantly, the s-Pt/1T′-MoS₂ can work at 1,500 mA cm⁻2 for 240 h without obvious degradation, exhibiting great potential in the practical water splitting. Density functional theory (DFT) calculations reveal that the s-Pt adsorbed on the top site of Mo on the 1T′-MoS₂ NSs exhibits a nearly thermoneutral hydrogen adsorption free energy, which could contribute to the superior HER performance of s-Pt/1T′-MoS₂.

In a first aspect, provided herein is a single-atomically dispersed metal/two-dimensional transition-metal dichalcogenide nanosheet hybrid (TMD NS hybrid) comprising a plurality of single-atomically dispersed metal atoms disposed on at least one surface of a transition-metal dichalcogenide nanosheet (TMD NS), wherein the transition-metal dichalcogenide nanosheet is uniformly crystalline.

In certain embodiments, each of the plurality of single-atomically dispersed metal atoms is ruthenium, rhodium, palladium, silver, osmium, iridium, platinum, gold, iron, cobalt, nickel, copper, zinc, cadmium, indium, tin, antimony, lead, bismuth, or other metal atoms.

In certain embodiments, each of the plurality of single-atomically dispersed metal atoms is platinum, gold, nickel, iridium, silver, tin, bismuth, or copper.

In certain embodiments, the TMD NS comprises MoS₂, MoSe₂, MoTe₂, WS₂, WSe₂, MOTe₂, WTe₂, TiS₂, TiSe₂, TaS₂, TaSe₂, VS₂, VSe₂, NbS₂, NbSe₂, ReS₂, ReSe₂, MoS_(2(1-A))Se_(2A), or WS_(2(1-A))Se_(2A), wherein A is 0-1.

In certain embodiments, the crystal phase of the transition-metal dichalcogenide is 1T′ phase.

In certain embodiments, the TMD NS comprises 1T′-MoS₂, 1T′-MoSe₂, 1T′-MoSSe, or 1T′-WS₂.

In certain embodiments, the TMD NS comprises 1T′-MoS₂.

In certain embodiments, each of the plurality of single-atomically dispersed metal atoms is platinum, gold, nickel, iridium, silver, tin, bismuth, or copper and the TMD NS comprises 1T′-MoS₂, 1T′-MoSe₂, 1T′-MoSSe, or 1T′-WS₂.

In certain embodiments, each of the plurality of single-atomically dispersed metal atoms is platinum and the TMD NS comprises 1T′-MoS₂.

In certain embodiments, the plurality of single-atomically dispersed metal atoms is present in the TMD NS hybrid at a weight percentage of 12.2 wt % or less.

In certain embodiments, the plurality of single-atomically dispersed metal atoms are present in the TMD NS hybrid at a weight percentage of 10.0 wt % or less.

In certain embodiments, each of the plurality of single-atomically dispersed metal atoms is gold or platinum; the TMD NS comprises 1T′-MoS₂; and the plurality of single-atomically dispersed metal atoms are present in the TMD NS hybrid at a weight percentage of 10.0 wt % or less.

In a second aspect, provided herein is a method of preparing the TMD NS hybrid of the first aspect, the method comprising: contacting a TMD NS with a plurality of single-atomically dispersed metal atom precursors in the presence of a reducing agent thereby forming the TMD NS hybrid, TMD NS is uniformly crystalline.

In certain embodiments, each of the plurality of single-atomically dispersed metal atom precursors are metal salts comprising at least one metal atom.

In certain embodiments, the plurality of single-atomically dispersed metal atom precursors is selected from the group consisting of M₂PtX₄, M₂PtX₆, M₂IrX₆, MAuX₄, SnY₃, BiY₃, CuY₂, AgY, NiY, wherein X is halide and Y is nitrate, cyanide, formate, acetate, or acetylacetonate; and M is hydrogen, lithium, sodium, potassium, or cesium.

In certain embodiments, the reducing agent is ascorbic acid, sodium citrate, metal hydride, H₂, hydrazine, alcohol, organolithium, electrochemical reduction, or photoreduction optionally in the presence of an additional reducing agent.

In certain embodiments, the plurality of single-atomically dispersed metal atom precursors is K₂PtCl₄, H₂IrCl₆, HAuCl₄, SnCl₃, BiCl₃, CuCl₂, AgNO₃, or NiNO₃, and the reducing agent is photoreduction in the presence of an alcohol or chemical reduction by using n-butyllithium as reducing agent.

In certain embodiments, the plurality of single-atomically dispersed metal atoms are present in the TMD NS hybrid at a weight percentage of 10.0 wt % or less.

In a third aspect, provided herein is an electrode comprising a base electrode and the TMD NS hybrid of the first aspect, wherein the base electrode is a planar electrode, including the glassy carbon electrode, a graphite electrode, an indium tin oxide (ITO) electrode, a fluorine doped tin oxide (FTO) electrode, a gas diffusion electrode (GDE), carbon paper electrode, carbon fiber electrode, polycarbonate track etch (PCTE)-based electrode, or titanium-based electrode.

In a fourth aspect, provided herein is an electrochemical cell comprising: a cathode comprising the TMD NS hybrid of the first aspect; an anode; and an electrolyte.

In a fifth aspect, provided herein is a method of producing hydrogen gas, the method comprising reducing a proton source at the cathode of the electrochemical cell of the fourth aspect thereby producing hydrogen gas, wherein the proton source is water optionally comprising an acid.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and features of the present disclosure will become apparent from the following description of the disclosure, when taken in conjunction with the accompanying drawings.

FIG. 1 depicts (A) Scanning electron microscopy (SEM) image of an exemplary exfoliated 1T′-MoS₂ NS, (B) Atomic force microscope (AFM), and (C) Transmission electron microscopy (TEM) image of an exemplary exfoliated 1T′-MoS₂ NS, and (D) High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images of an exemplary exfoliated 1T′-MoS₂ NS. Inset: the corresponding fast Fourier transform (FFT) pattern.

FIG. 2 depicts (A) TEM image of s-Pt/1T′-MoS₂. Inset: the corresponding selected area electron diffraction (SAED) pattern. (B) STEM image and the corresponding energy-dispersive X-ray spectroscopy (EDS) elemental mappings of s-Pt/1T′-MoS₂. (C) Atomic-resolution HAADF-STEM image of s-Pt/1T′-MoS₂ showing the isolated Pt atomically dispersed on 1T′-MoS₂ NS. Three kinds of single-atomically dispersed Pt, denoted as Pt_(sub), Pt_(ads-S) and Pt_(ads-Mo), respectively, are enclosed by the white dotted rectangles. (D)-(F) Simulated atomic structures (D₁-F₁), the corresponding simulated STEM images (D₂-F₂), and the intensity profiles (D₃-F₃) of Pt_(sub) (D) Pt_(ads-S) (E) and Pt_(ads-Mo) (F), respectively. The dashed curves of the simulated intensity profiles in (D₃-F₃) are taken from the corresponding white dotted rectangles in the simulated STEM images (D₂-F₂). The histograms of the experimental intensity profiles in (d₃-f₃) are taken from the corresponding white dotted rectangles in (C).

FIG. 3 depicts (A) Pt L₃-edge X-ray absorption near edge structure (XANES), and (B) Fourier-transformed Extended X-ray absorption fine structure (EXAFS) spectra of s-Pt/1T′-MoS₂, commercial PtS₂ powder, and Pt foil. (C) Pt 4f, and d, Mo 3d XPS spectra of s-Pt/1T′-MoS₂. The hollow points and the solid lines in (C) and (D) show the experimental XPS data and the corresponding deconvoluted spectra, respectively.

FIG. 4 depicts (A) HER polarization curves of s-Pt/1T′-MoS₂, 1T′-MoS₂ NSs, and commercial Pt/C electrocatalysts loaded on the glassy carbon electrode (GCE). The weights of 1T′-MoS₂ NSs, and s-Pt/1T′-MoS₂ were kept the same (0.1 mg cm⁻²). The mass loadings of Pt were kept the same (0.01 mg cm⁻²) for s-Pt/1T′-MoS₂, and Pt/C. (B) Tafel plots of the catalysts derived from (A). (C) Mass activities of s-Pt/1T′-MoS₂ and commercial Pt/C. (D) Long-term stability test for s-Pt/1T′-MoS₂. The polarization curves were recorded before and after 10,000 potential cycles. Inset: the chronoamperometric test of s-Pt/1T′-MoS₂, showing that the current density remains stable over 30 h. (E) Measurements of the HER activity and stability of s-Pt/1T′-MoS₂ loaded on carbon fiber paper in an H-type cell with a Pt mass loading of 0.0175 mg cm⁻². The polarization curves of s-Pt/1T′-MoS₂ were recorded initially and after the 240-h test under current density of 1,500 mA cm⁻². (F) Time-dependent potential curve of s-Pt/1T′-MoS₂ loaded on carbon fiber paper in an H-type cell under a current density of 1,500 mA cm⁻². All the HER measurements were conducted in 0.5 M H₂SO₄ aqueous solution, and all the current densities were normalized by the geometric area of the electrode.

FIG. 5 depicts (A) SEM image of an exemplary 1T′-MoSe₂ NSs sample, (B) AFM image of an exemplary 1T′-MoSe₂ NSs sample, (C) TEM image of an exemplary 1T′-MoSe₂ NSs sample, and (D) the SAED pattern of an exemplary 1T′-MoSe₂ NSs sample.

FIG. 6 depicts (A) SEM image of an exemplary 1T′-MoSSe NSs sample, (B) AFM image of an exemplary 1T′-MoSSe NSs sample, (C) TEM image of an exemplary 1T′-MoSSe NSs sample, and (D) SAED pattern of an exemplary 1T′-MoSSe NSs sample.

FIG. 7 depicts (A) SEM image of an exemplary 1T′-WS₂ NSs sample, (B) AFM image of an exemplary 1T′-WS₂ NSs sample, (C) TEM image of an exemplary 1T′-WS₂ NSs Ss sample, and (D) the SAED pattern image of an exemplary 1T′-WS₂ NSs sample.

FIG. 8 depicts (A) HAADF-STEM image of an exemplary s-Au/1T′-MoS₂ sample, (B) Raman spectrum of an exemplary s-Au/1T′-MoS₂ sample, (C) Mo 3d XPS spectrum of an exemplary s-Au/1T′-MoS₂ sample, (D) EDS mapping of an exemplary s-Au/1T′-MoS₂ sample. Inset in a: the corresponding FFT pattern.

FIG. 9 depicts (A) HAADF-STEM image of an exemplary s-Pt/1T′-WS₂ sample, (B) EDS mapping of an exemplary s-Pt/1T′-WS₂ sample. Inset in A: the corresponding FFT pattern.

FIG. 10 depicts (A) TEM image of an exemplary s-Ni/1T′-WS₂ sample, (B) HAADF-STEM image of an exemplary s-Ni/1T′-WS₂ sample, (C) EDS mapping of an exemplary s-Ni/1T′-WS₂ sample. Inset in B: the corresponding FFT pattern.

FIG. 11 depicts (A) TEM image of an exemplary s-Ir/1T′-WS₂ sample, (B) HRTEM image of an exemplary s-Ir/1T′-WS₂ sample, (C) EDS mapping of an exemplary s-Ir/1T′-WS₂ sample. Inset in B: the corresponding FFT pattern.

FIG. 12 depicts (A) TEM image of an exemplary s-Ag/1T′-WS₂ sample, (B) HRTEM image of an exemplary s-Ag/1T′-WS₂ sample, (C) EDS mapping of an exemplary s-Ag/1T′-WS₂ sample. Inset in B: the corresponding FFT pattern.

FIG. 13 depicts STEM and EDS mapping of an exemplary s-Sn/1T′-MoS₂ sample.

FIG. 14 depicts STEM and EDS mapping of an exemplary s-Bi/1T′-MoS₂ sample.

FIG. 15 depicts STEM and EDS mapping of an exemplary s-Sn-Bi/1T′-MoS₂ sample. Scale: 1 μm.

FIG. 16 depicts STEM and EDS mapping of an exemplary s-Sn—Bi—Cu/1T′-MoS₂ sample.

FIG. 17 depicts polarization curves of the proton-exchange membrane (PEM) electrolyser using s-Pt/1T′-MoS₂ as cathode catalysts in comparison with the commercial Pt/C catalysts.

FIG. 18 depicts the chronopotentiometry curve of the PEM electrolyser using s-Pt/1T′-MoS₂ as cathode catalysts.

FIG. 19 depicts optical microscopy of obtained 1T′-MoS₂ NSs.

FIG. 20 depicts (A) HAADF-STEM image of s-Pt/1T′-MoS₂. (B) The corresponding line scan intensity profile obtained from the white dotted rectangle in (A).

FIG. 21 depicts atomic structural models of Pt_(sub), Pt_(ads-S), and Pt_(ads-Mo) showing the bonding environment of Pt. (A) Top view, and (B) side view of Pt_(sub). The Pt_(sub) is located in the 1T′-MoS₂ lattice, substituting the Mo site, and coordinated with six surrounding S atoms. (C) Top view, and (D) side view of Pt_(ads-S) is The Pt_(ads-S) is located on the top site of S in 1T′-MoS₂ and coordinated with two adjacent S atoms. (E) Top view, and (F) side view of Pt_(ads-Mo). The Pt_(ads-Mo) is located on the top site of Mo in 1T′-MoS₂ and coordinated with three adjacent S atoms.

FIG. 22 depicts fitting curves for the EXAFS profiles of the Fourier transform at Pt L₃-edge. (A)-(C), Pt L₃-edge EXAFS spectra and the corresponding fitting curves of s-Pt/1T′-MoS₂ (A), commercial PtS₂ powder (B), and Pt foil (C). The fitting results are summarized in Table 2 (FIG. 38 ).

FIG. 23 depicts Pt L₃-edge XANES spectra and the structural model of s-Pt/1T′-MoS₂. (A) Pt L₃-edge EXAFS spectrum of s-Pt/1T′-MoS₂ and the simulated spectrum using (B) the simulated DFT model with the coexistence of Pt_(sub), Pt_(ads-S), and Pt_(ads-Mo) on 1T′-MoS₂ (denoted as Pt_(sub)+Pt_(ads-S)+Pt_(ads-Mo)).

FIG. 24 depicts Raman characterization of synthesized samples. Raman spectra of 1T′-MoS₂ NSs and s-Pt/1T′-MoS₂.

FIG. 25 depicts characterizations of s-Pt-4/1T′-MoS₂ with Pt loading of 4.3 wt %. (A)-(D), HAADF-STEM image (A) EDS mapping (B) Raman spectrum (C), and Mo 3d XPS spectrum (D) of the s-Pt-4/1T′-MoS₂. Inset in a: the corresponding FFT pattern of s-Pt-4/1T′-MoS₂.

FIG. 26 depicts characterizations of s-Pt-6/1T′-MoS₂ with Pt loading of 6.4 wt %. (A)-(D), HAADF-STEM image (A) EDS mapping (B) Raman spectrum (C), and Mo 3d XPS spectrum (D) of the s-Pt-6/1T′-MoS₂. Inset in a: the corresponding FFT pattern of s-Pt-6/1T′-MoS₂.

FIG. 27 depicts Characterizations of PtNPs-12/1T′-MoS₂ with the Pt loading of 12.2 wt %. (A)-(D), HAADF-STEM image (A) EDS mapping (B) Raman spectrum (C) and Mo 3d XPS spectrum (D) of the PtNPs-12/1T′-MoS₂. Inset in (A) the corresponding FFT pattern of PtNPs-12/1T′-MoS₂.

FIG. 28 depicts Characterizations of PtNPs-15/1T′-MoS₂ with the Pt loading of 15.5 wt %. (A)-(D), HAADF-STEM image (A) EDS mapping (B) Raman spectrum (C) and Mo 3d XPS spectrum (D) of the PtNPs-15/1T′-MoS₂. Inset in a: the corresponding FFT pattern of PtNPs-15/1T′-MoS₂.

FIG. 29 depicts Pt 4f XPS spectra of s-Pt-4/1T′-MoS₂, s-Pt-6/1T′-MoS₂, s-Pt/1T′-MoS₂, PtNPs-12/1T′-MoS₂, and PtNPs-15/1T′-MoS₂.

FIG. 30 depicts the characterization of the commercial Pt/C. (A) TEM, (B) high-resolution TEM (HRTEM), and (C) STEM images and the corresponding EDS mappings of the commercial Pt/C.

FIG. 31 depicts CO stripping voltammograms of commercial Pt/C (A) and s-Pt/1T′-MoS₂ (B) in 0.5 M H₂SO₄ aqueous solution. The solid curves indicate the stripping of CO in the first scan cycle, and the dashed curves represent the second scan cycle after the stripping of CO.

FIG. 32 depicts the characterization of s-Pt/1T′-MoS₂ after 10,000 cycles of the HER test. (A) TEM, (B) HRTEM, and (C) HAADF-STEM image of s-Pt/1T′-MoS₂. Inset in (B): the corresponding FFT pattern of s-Pt/1T′-MoS₂. (D) STEM image and the corresponding EDS mappings of s-Pt/1T′-MoS₂.

FIG. 33 depicts HER performances of s-Pt/1T′-MoS₂. (A) Polarization curves and (B) Tafel plots (derived from (A)) of s-Pt/1T′-MoS₂ tested on GCE in N₂-saturated 0.5 M H₂SO₄ aqueous solution and on rotating disk electrode (RDE) at a rotation rate of 1,600 r.p.m. in H₂-saturated 0.5 M H₂SO₄ aqueous solution.

FIG. 34 depicts HER performances of s-Pt-4/1T′-MoS₂, s-Pt-6/1T′-MoS₂, s-Pt/1T′-MoS₂, PtNPs-12/1T′-MoS₂, PtNPs-15/1T′-MoS₂ and commercial Pt/C catalysts. The catalysts were coated on RDEs and tested at a rotation rate of 1,600 r.p.m. in H₂-saturated 0.5 M H₂SO₄ aqueous solution.

FIG. 35 depicts DFT calculation results of s-Pt/1T′-MoS₂ for HER. (A)-(C) DFT models of isolated Pt_(ads-S) (A) isolated Pt_(ads-Mo) (B) and Pt_(sub)+P_(ads-S)+Pt_(ads-Mo) (C) used for the hydrogen adsorption free energy and the projected density of states (PDOS) calculation. Pt′_(ads-S), and Pt′_(ads-Mo) in c refer to the Pt_(ads-S) and Pt_(ads-Mo) in the model of Pt_(sub)+Pt_(ads-S)+Pt_(ads-Mo), respectively. The model Pt_(sub)+Pt_(ads-S)+Pt_(ads-Mo) is established based on the experimental and simulation results shown in FIG. 23B, Calculated free energy diagrams for HER. (E) PDOS of the d-band of Pt atoms with the corresponding configurations as shown in (A)-(C). The d-band center (E d) values are shown in e. The black dotted line in e indicates the Fermi level.

FIG. 36 depicts polarization curves of s-Pt/1T′-MoS₂ and commercial Pt/C loaded on carbon fiber paper and tested in H-type cell.

FIG. 37 depicts Table 1 showing ICP-OES results and the corresponding Pt mass loadings in the as-synthesized samples.

FIG. 38 depicts Table 2 showing fitting parameters of Pt L₃-edge EXAFS curves showing the structure parameters around Pt in the s-Pt/1T′-MoS₂, commercial PtS₂ powder, and Pt foil.

FIG. 39 depicts Table 3 showing the comparison of HER activities of some representative reported Pt-based catalysts tested in acidic media.

FIG. 40 depicts Table 4 showing the comparison of mass activities of s-Pt/1T′-MoS₂ and commercial Pt/C.

FIG. 41 depicts Table 5 showing the comparison of Q_(CO), number of active sites, ECSA and TOF values of s-Pt/1T′-MoS₂ and the commercial Pt/C catalysts.

FIG. 42 depicts Table 6 showing the comparison of high-current-density HER performance of the catalysts described herein with the previously reported catalysts in 0.5 M H₂SO₄ aqueous solution.

DETAILED DESCRIPTION Definitions

Throughout the application, where compositions are described as having, including, or comprising specific components, or where processes are described as having, including, or comprising specific process steps, it is contemplated that compositions of the present teachings can also consist essentially of, or consist of, the recited components, and that the processes of the present teachings can also consist essentially of, or consist of, the recited process steps.

In the application, where an element or component is said to be included in and/or selected from a list of recited elements or components, it should be understood that the element or component can be any one of the recited elements or components, or the element or component can be selected from a group consisting of two or more of the recited elements or components. Further, it should be understood that elements and/or features of a composition or a method described herein can be combined in a variety of ways without departing from the spirit and scope of the present teachings, whether explicit or implicit herein.

It should be understood that the order of steps or order for performing certain actions is immaterial so long as the present teachings remain operable. Moreover, two or more steps or actions may be conducted simultaneously.

The use of the singular herein includes the plural (and vice versa) unless specifically stated otherwise. In addition, where the use of the term “about” is before a quantitative value, the present teachings also include the specific quantitative value itself, unless specifically stated otherwise. As used herein, the term “about” refers to a ±10%, ±7%, ±5%, ±3%, ±1%, or ±0% variation from the nominal value unless otherwise indicated or inferred.

Provided herein is a TMD NS hybrid comprising a plurality of single-atomically dispersed metal atoms disposed on at least one surface of a TMD NS, wherein the TMD NS is uniformly crystalline. In certain embodiments, the TMD NS is uniformly 1T′ crystal phase.

The TMD NS can comprise MoS₂, MoSe₂, MoTe₂, WS₂, WSe₂, MoTe₂, WTe₂, TiS₂, TiSe₂, TaS₂, TaSe₂, VS₂, VSe₂, NbS₂, NbSe₂, ReS₂, ReSe₂, MoS_(2(1-A))Se_(2A), or WS_(2(1-A))Se_(2A), wherein A is 0-1. In certain embodiments, the TMD NS comprises MoS₂, MoSe₂, MoSSe, or WS₂.

The TMD NS has an unconventional crystalline structure. In certain embodiments, the crystalline structure of the TMD NS is 1T′ crystal phase, such as 1T′-MoS₂, 1T′-MoSe₂, 1T′-MoSSe, or 1T′-WS₂. In certain embodiments, the crystalline regions of the TMD NS may account for greater than 90% by volume of the TMD NS. In other embodiments, the crystalline regions may account for greater than 92%, 95%, 97%, 98%, 99%, or 99.9% of the volume of the TMD NS. In certain embodiments, the crystalline regions may account for a volume of the TMD NS in the range of 70% to 100%, 80% to 100%, 90% to 100%, 90% to 99%, 95% to 100%, 95% to 99%, 96% to 100%, 96% to 99%, 97% to 100%, 97% to 99%, 98% to 100%, 98% to 99%, 99% to 100%, 99.9 to 100%, or any value or range of values within those ranges.

The TMD NS hybrid can comprise between 1-100 layers of the TMD NSs. In certain embodiments, the TMD NS hybrid has 1-90 layers, 1-80 layers, 1-70 layers, 1-60 layers, 1-50 layers, 1-40 layers, 1-30 layers, 1-25 layers, 1-20 layers, 1-15 layers, 1-10 layers, 1-5 layers, 2-5 layers, 2-30 layers, 2-25 layers, 2-20 layers, 2-15 layers, 2-11 layers, 2-5 layers, 3-30 layers, 3-25 layers, 3-20 layers, 3-15 layers, 3-11 layers, 1-5 layers, 3-5 layers, 1-3 layers, 1-2 layers, or 2-3 layers of the TMD NSs.

The selection of the type of plurality of single-atomically dispersed metal atoms is not particularly limited. Any metal can be used in connection with the TMD NS hybrids described herein. In certain embodiments, each of the plurality of single-atomically dispersed metal atom is ruthenium, rhodium, palladium, silver, osmium, iridium, platinum, gold, iron, cobalt, nickel, copper, zinc, cadmium, indium, tin, antimony, lead, bismuth, or other metal atoms. In certain embodiments, each of the plurality of single-atomically dispersed metal atoms is platinum, gold, nickel, iridium, silver, tin, bismuth, or copper.

The plurality of single-atomically dispersed metal atoms can exist in any oxidation state, such as +1, +2, +3, +4 and mixtures thereof.

In certain embodiments, the TMD NS hybrid comprises less than 5% wt, less than 4% wt, less than 3% wt, less than 2% wt, less than 1% wt, less than 0.5% wt, less than 0.1% wt, less than 0.01% wt, less than 0.001% wt, or an undetectable amount of nanoparticles comprising metal atoms.

The plurality of single-atomically dispersed metal atoms can be present in the TMD NS hybrid at a weight percentage of 12.2 wt % or less. In certain embodiments, the plurality of single-atomically dispersed metal atoms are present in the TMD NS hybrid at a weight percentage of 10.0 wt % or less, 9.5 wt % or less, 9.0 wt % or less, 8.5 wt % or less, 8.0 wt % or less, 7.5 wt % or less, 7.0 wt % or less, 6.5 wt % or less, 6.4 wt % or less, 6.0 wt % or less, 5.5 wt % or less, 5.0 wt % or less, 4.5 wt % or less, 4.3 wt % or less, 4.0 wt % or less, 3.5 wt % or less, 3.0 wt % or less, 2.5 wt % or less, 2.0 wt % or less, 1.5 wt % or less, 1.0 wt % or less, 0.5 wt % or less, or 0.1 wt % or less. In certain embodiments, the plurality of single-atomically dispersed metal atoms are present in the TMD NS hybrid at a weight percentage of 0.1 to 10 wt %, 0.5 to 10 wt %, 1.0 to 10 wt %, 1.5 to 10 wt %, 2.0 to 10 wt %, 2.5 to 10 wt %, 3.0 to 10 wt %, 3.5 to 10 wt %, 4.0 to 10 wt %, 4.3 to 10 wt %, 4.3 to 6.4 wt %, 4.5 to 10 wt %, 5.0 to 10 wt %, 5.5 to 10 wt %, 6.0 to 10 wt %, 6.4 to 10 wt %, 6.5 to 10 wt %, 7.0 to 10 wt %, 7.5 to 10 wt %, 8.0 to 10 wt %, 8.5 to 10 wt %, 9.0 to 10 wt %, or 9.5 to 10 wt %.

The phrase “disposed on at least one surface of a TMD NS” is intended to encompass one or more different types of configurations in which each of the plurality of single-atomically dispersed metal atoms is adsorbed to a surface of the TMD NS, such as via bonded and/or non-bonded interactions with the molybdenum atom or via bonded and/or non-bonded interactions with the sulfur atom, in the TMD NS lattice, e.g., substituting the transition metal atom at the transition metal site of the lattice, and coordinated with six surrounding chalcogenide atoms, or a combination thereof.

The present disclosure also provides a method for preparing a TMD NS hybrid described herein, the method comprising: contacting a TMD NS with a plurality of single-atomically dispersed metal atom precursors in the presence of a reducing agent thereby forming the TMD NS hybrid, wherein the TMD NS is uniformly crystalline. In certain embodiments, the TMD NS is uniformly 1T′ crystal phase.

The TMD NS hybrid described herein can be prepared by reducing the plurality of single-atomically dispersed metal atom precursors in the presence of the TMD NS. Methods for reducing the single-atomically dispersed metal atom precursors are well known in the art and include, but are not limited to, chemical reduction, electrochemical reduction, and photoreduction. The selection for the appropriate method for reducing the single-atomically dispersed metal atom precursor and the conditions for accomplishing the same are well within the skill of a person of ordinary skill in the art.

The single-atomically dispersed metal atom precursor can be a metal salt comprising the metal atom in an oxidized state. In certain embodiments, the oxidized state of the metal atom is +1, +2, +3, +4, +5, +6, +7, or +8. The single-atomically dispersed metal atom precursor can comprise a metal selected from the group consisting of ruthenium, rhodium, palladium, silver, osmium, iridium, platinum, gold, iron, cobalt, nickel, copper, zinc, cadmium, indium, tin, antimony, lead, bismuth, or other metal atoms.

The single-atomically dispersed metal atom precursor can comprise any anion or combination of anions. In certain embodiments, the metal salt single-atomically dispersed metal atom precursor comprises chloride, bromide, iodide, cyanide, perchlorate, carbonate, bicarbonate, sulfate, phosphate, monohydrogen phosphate, dihydrogen phosphate, metaphosphate, pyrophosphate, acetate, maleate, fumarate, formate, malonate, oxalate, lactate, tartrate, citrate, gluconate, mesylate, besylate, tosylate, succinate, and salicylate sulfate, sulfite, bisulfate, bisulfite, nitrate, nitrite, or a combination thereof.

The metal salt single-atomically dispersed metal precursor must be charged balanced. The ratio of the metal to the anion can be represented by the formula (A^(t+))_(U)(Bu^(u−))_(T) wherein t represents the charge of the metal, U represents the charge of the anion, U is equal to the absolute value of the charge of the anion and T is equal to the absolute value of the charge of the metal. For example, when the metal has a charge of 3+ and the anion is Cl—, which has a charge of −1, the charged balance formula would be (A³⁺)₁(Cl⁻)₃.

In certain embodiments, the single-atomically dispersed metal precursor is selected from the group consisting of M₂PtX₄, M₂PtX₆, M₂IrX₆, MA_(u)X₄, SnY₃, BiY₃, CuY₂, AgY, NiY, wherein X is halide and Y is nitrate, cyanide, formate, acetate, or acetylacetonate; and M is hydrogen, lithium, sodium, potassium, cesium, or a combination thereof. In certain embodiments, the single-atomically dispersed metal precursor is K₂PtCl₄, H₂IrCl₆, HAuCl₄, SnCl₃, BiCl₃, CuCl₂, AgNO₃, or NiNO₃.

The reducing agent can be ascorbic acid, sodium citrate, metal hydride, H₂, hydrazine, alcohol, organolithium, electrochemical reduction, or photoreduction optionally in the presence of an additional reducing agent. The metal hydride reducing agents include, but are not limited to, borohydride, such as NaBH₄, KBH₄, ZnBH₄, NaBH₃CN, and Li-s-Bu₃BH; aluminum and tin compounds, such as lithium aluminum hydride (LiAlH₄), diisobutylalurniniurn hydride (DIPAL-H) and SnCl₂/pyridine: borane (BH₃) or borane complexes, such as B₂H₆ and dimethylamine borane. In certain embodiments, the reducing agent is photoreduction in the presence of an alcohol, such as ethanol; or the reducing agent is n-butyllithium.

The step of contacting a TMD NS with the plurality of single-atomically dispersed metal atom precursors in the presence of a reducing agent can be conducted in any solvent in which the plurality of single-atomically dispersed metal atom precursors and the reducing agent are at least partially soluble. In certain embodiments, the solvent is a polar protic solvent. In certain embodiments, the solvent is water, an alcohol, an amine, and mixtures thereof. The alcohol can be a C₁-C₆ alkyl alcohol, such as methanol, ethanol, 1-propanol, 2-propanol, n-butyl alcohol, sec-butanol, tert-butanol, ethylene glycol, propylene glycol, and mixtures thereof. In certain embodiments, the solvent is a mixture of water and ethanol or oleylamine.

The present disclosure also provides an electrode comprising a base electrode and the TMD NS hybrid described herein. In certain embodiments, the TMD NS hybrid is coated on the surface of the base electrode. The base electrode can be an inert electrode such as a GCE, a graphite electrode, an indium tin oxide (ITO) electrode, a fluorine doped tin oxide (FTO) electrode, carbon paper electrode, carbon fiber electrode, polycarbonate track etch (PCTE)-based electrode, or titanium-based electrode. In certain embodiments, the electrode is a cathode.

The present disclosure also provides an electrochemical cell comprising: a cathode comprising the TMD NS hybrid described herein; an anode; and an electrolyte. In certain embodiments, the cathode further comprises a base electrode, wherein the base electrode can be an inert electrode such as a GCE, a graphite electrode, an indium tin oxide (ITO) electrode, a fluorine doped tin oxide (FTO) electrode, carbon paper electrode, carbon fiber electrode, polycarbonate track etch (PCTE)-based electrode, or titanium-based electrode.

The present disclosure also provides a device for hydrogen production by water electrolysis, wherein the device comprises the electrode described herein.

Also provided herein is a method of producing hydrogen gas, the method comprising reducing a proton source at the cathode of the electrochemical cell described herein thereby producing hydrogen gas, wherein the proton source is water optionally comprising an acid. In certain embodiments, the acid is sulfuric acid.

The TMD NS hybrid described herein can also be used as a catalyst in various reactions. Exemplary reactions include, but are not limited to, CO₂ reduction, nitrogen reduction, nitrite reduction, nitrate reduction, aldehyde oxidation, and the like.

Electrochemical Intercalation and Characterization of 1T′-MoS₂ NSs

Briefly, the tetraheptylammonium bromide molecules were first intercalated into the prepared K,MoS₂ crystals in an electrochemical cell. After the exfoliation, 1T′-MoS₂ NSs (FIG. 1A) with a thickness of 1.4±0.4 nm (FIG. 1B) and lateral size of up to several micrometers (FIG. 1A and 1C, FIG. 19 ) were prepared. As shown in the aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image (FIG. 1D), the zigzag chains with the shortest Mo—Mo distance of 2.78 Å are observed, which is consistent with the theoretical 1T′-MoS₂ structure. Moreover, the corresponding fast Fourier transform (FFT) pattern (inset of FIG. 1D) confirms the distorted octahedral coordinated structure and the high crystallinity of the obtained 1T′-MoS₂ NSs.

Synthesis of Single-Atomically Dispersed Pt on 1T′-MoS₂ NSs

The high-quality 1T′-MoS₂ NSs can be used as ideal templates to grow metals to construct metal/MoS₂ hybrids. In this work, Pt was grown on the MoS₂ surface through a photoreduction process.

Surprisingly, when 1T′-MoS₂ NSs were used as templates, the grown Pt was single-atomically dispersed on the 1T′-MoS₂ with a loading of ˜10.0 wt % (FIG. 37 , Table 1). No PtNPs can be observed on the surface of 1T′-MoS₂ (FIG. 2A), and no diffraction spots assigned to PtNPs can be observed in the SAED pattern (inset of FIG. 2A). The EDS elemental mapping images (FIG. 2B) confirm the presence and homogeneous dispersion of Pt on the 1T′-MoS₂ NSs. Moreover, the aberration-corrected HAADF-STEM was used to characterize the atomic structure of the s-Pt/1T′-MoS₂. As shown in FIG. 20A, Pt atoms exhibit much stronger brightness than the Mo atoms due to the Z-contrast nature, and both of them can be easily distinguished in the line scan intensity profile (FIG. 20B). In the HAADF-STEM image in FIG. 2C, three kinds of single-atomically dispersed Pt on 1T′-MoS₂ can be identified based don their locations, i.e., Pt substituting Mo site (Pt_(sub)), Pt adsorbed on the top site of S (Pt_(ads-S)), and Pt adsorbed on the top site of Mo (Pt_(ads-Mo)). The optimized DFT models (FIG. 2D ₁-F₁ and FIG. 21 ) display the detailed bonding environments of Pt_(sub), Pt_(ads-S), and Pt_(ads-Mo). The simulated intensity profiles (dashed curves in FIG. 2D ₃-F₃) taken from the simulated STEM images (FIG. 2D ₂-F₂) based on the DFT models (FIG. 2D ₁-F₁) show good agreement with the experimental intensity profiles (histograms in FIG. 2D ₃-F₃) which are taken from the corresponding white dotted rectangles in FIG. 2C.

The X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS) profiles were performed to investigate the electronic structure and coordination of the Pt in s-Pt/1T′-MoS₂. As shown in FIG. 3A, the height of the white line of Pt L₃-edge XANES of s-Pt/1T′-MoS₂ is closer to that of PtS₂ but significantly higher than that of Pt foil, indicating the Pt species in the s-Pt/1T′-MoS₂ are dominantly in the oxidation state. The Fourier-transformed EXAFS of s-Pt/1T′-MoS₂ exhibits a dominant peak at 1.87 Å (FIG. 3B), which can be attributed to the Pt-S contribution. The fitting results (FIG. 22 and FIG. 38 , Table 2) demonstrate that the Pt-S bond has an average bond length of 2.30 Å and an average coordination number of 4.2 in the s-Pt/1T′-MoS₂. No metallic peak attributed to the Pt—Pt bond could be assigned, further confirming the single-atomically dispersed Pt on 1T′-MoS₂ NSs. In addition, the good agreement between the experimental and simulated XANES curves (FIG. 23 ) affirms three kinds of Pt atoms (Pt_(sub), Pt_(ads-S), and Pt_(ads-Mo)) coexisting in the s-Pt/1T′-MoS₂, which is consistent with the HAADF-STEM analysis (FIG. 2 ).

The Pt L₃-edge EXAFS spectrum of s-Pt/1T′-MoS₂ (FIG. 3A and FIG. 23A) shows that the Pt is single-atomically dispersed on 1T′-MoS₂ NSs. The STEM analysis (FIG. 2 ) and the DFT calculation results (FIG. 21 ) indicate that the s-Pt/1T′-MoS₂ contains three kinds of Pt atomic structures, i.e., Pt_(sub), Pt_(ads-S), and Pt_(ads-Mo). After a systematic screening, a DFT model containing Pt_(sub), Pt_(ads-S), and Pt_(ads-Mo) is established, which is denoted as Pt_(sub)+Pt_(ads-S)+Pt_(ads-Mo) (FIG. 23 ). The simulated Pt L₃-edge XANES spectrum based on this model matches well with the experimental curve of s-Pt/1T′-MoS₂ (FIG. 23 ). This result reveals the coexistence of Pt_(sub), Pt_(ads-S), and Pt_(ads-Mo) in the prepared s-Pt/1T′-MoS₂, which is consistent with the HAADF-STEM analysis (FIG. 2 ).

XPS was further used to reveal the valance states of Pt and Mo in the synthesized Pt-MoS₂ hybrids. As shown in FIG. 3C, the fitted peaks of Pt 4f_(7/2) located at 72.5 eV, 73.5 eV, and 74.8 eV can be assigned to Pt^(δ+), Pt²⁺, and Pt⁴⁺, respectively, suggesting the formation of Pt-S bond. These XPS results are in accordance with the aforementioned XANES data (FIG. 3A-B). The valance states of MoS₂ templates after the growth of Pt were also characterized. As shown in FIG. 3D, the Mo 3d_(5/2) and Mo 3d_(3/2) peaks of s-Pt/1T′-MoS₂ can be well assigned to the 1T′-MoS₂. These results indicate that no phase transition occurred in the 1T′-MoS₂ templates after the growth of Pt, which is also confirmed by Raman spectra (FIG. 24 ).

All the Raman peaks of s-Pt/1T′-MoS₂ also shift to higher wavenumbers compared to the 1T′-MoS₂ NSs (FIG. 24 ). Importantly, the characteristic E_(2g) ¹ peak of 2H-MoS₂ located at 382.8 cm⁻¹ cannot be observed, indicating that no phase transition from 1T′ to 2H phase occurred after the growth of Pt on 1T′-MoS₂.

Moreover, by simply changing the amount of Pt precursors, the loading (as determined by the ICP-OES, FIG. 37 , Table 1) and structure of Pt can be easily controlled during synthesis. Specifically, s-Pt can also be grown on 1T′-MoS₂ NSs with lower Pt loadings of 4.3 wt % (s-Pt-4/1T′-MoS₂, FIG. 25 ) and 6.4 wt % (s-Pt-6/1T′-MoS₂, FIG. 26 ). When the Pt loading increases over 10.0 wt %, PtNPs can be formed in Pt loadings of 12.2 wt % (PtNPs-12/1T′-MoS₂, FIG. 27 ) and 15.5 wt % (PtNPs-15/1T′-MoS₂, FIG. 28 ). The EDS mapping results (FIGS. 25B-28B) show the distribution of Pt nanostructures on the 1T′-MoS₂ NSs. Furthermore, the Raman spectra (FIGS. 25C-28C) and Mo 3d XPS spectra (FIGS. 25D-28D) confirm that no phase transition occurred in the 1T′-MoS₂ templates after the growth of Pt. Moreover, XPS was used to characterize the valance states of as-grown Pt nanostructures. Only Pt with oxidation states can be observed in the Pt 4f XPS spectra of s-Pt-4/1T′-MoS₂ and s-Pt-6/1T′-MoS₂ samples, which is consistent with that of the s-Pt/1T′-MoS₂ with Pt loading of 10.0 wt % (FIG. 29 ). In contrast, the strong Pt⁰ peaks can be observed in the Pt 4f XPS spectra of PtNPs-12/1T′-MoS₂ and PtNPs-15/1T′-MoS₂ (FIG. 29 ), confirming the formation of PtNPs at higher Pt loadings.

The as-grown Pt in the s-Pt-4/1T′-MoS₂ remained single-atomically dispersed on the 1T′-MoS₂, as shown in the HAADF-STEM image (FIG. 25A). The EDS mapping results (FIG. 25B) show the distribution of Pt, Mo, and S elements in the synthesized s-Pt-4/1T′-MoS₂. The FFT pattern (inset of FIG. 25A), Raman spectrum (FIG. 25C) and Mo 3d XPS spectrum (FIG. 25D) confirm that no phase transition occurred in 1T′-MoS₂ after the growth of single-atomically dispersed Pt.

The as-grown Pt in the s-Pt-6/1T′-MoS₂ remained single-atomically dispersed on the 1T′-MoS₂, as shown in the HAADF-STEM image (FIG. 26A). The EDS mapping results (FIG. 26B) show the distribution of Pt, Mo, and S elements in the synthesized s-Pt-6/1T′-MoS₂. The FFT pattern (inset of FIG. 26A), Raman spectrum (FIG. 26C) and Mo 3d XPS spectrum (FIG. 26D) confirm that no phase transition occurred in 1T′-MoS₂ after the growth of single-atomically dispersed Pt.

As shown in FIG. 29 , only Pt with oxidation states (Pt^(δ+), Pt²⁺, Pt⁴⁺) can be observed in the Pt 4f XPS spectra of s-Pt-4/1T′-MoS₂, s-Pt-6/1T′-MoS₂ and s-Pt/1T′-MoS₂. When the Pt loadings increase to 12.2 wt % and 15.5 wt %, the strong Pt⁰ peaks are found in the Pt 4f XPS spectra of PtNPs-12/1T′-MoS₂ and PtNPs-15/1T′-MoS₂, indicating the formation of Pt nanoparticles. The XPS results are consistent with the aforementioned HAADF-STEM analyses (FIGS. 25-28 ).

The results reveal that the loading amount and structure of as-grown Pt can be well controlled on the 1T′-MoS₂ NSs.

When the loading of Pt further increased to 12.2 wt % in the PtNPs-12/1T′-MoS₂, Pt nanoparticles have been found on the 1T′-MoS₂, as shown in the HAADF-STEM image (FIG. 27A). The EDS mapping results (FIG. 27B) show the distribution of Pt, Mo, and S elements in the synthesized PtNPs-12/1T′-MoS₂. The FFT pattern (inset of FIG. 27A), Raman spectrum (FIG. 27C) and Mo 3d XPS spectrum (FIG. 27D) confirm that no phase transition occurred in 1T′-MoS₂ after the growth of Pt.

When the loading of Pt reached 15.5 wt % in the PtNPs-15/1T′-MoS₂, Pt nanoparticles were observed on the 1T′-MoS₂, as shown in the HAADF-STEM image (FIG. 28A). The EDS mapping results (FIG. 28B) show the distribution of Pt, Mo, and S elements in the synthesized PtNPs-15/1T′-MoS₂. The FFT pattern (inset of FIG. 28A), Raman spectrum (FIG. 28C) and Mo 3d XPS spectrum (FIG. 28D) confirm that no phase transition occurred in 1T′-MoS₂ after the growth of Pt.

Electrochemical Hydrogen Evolution Reaction

As a proof-of-concept application, the obtained s-Pt/1T′-MoS₂ was used for the hydrogen evolution reaction (HER), which was tested in 0.5 M H₂SO₄ aqueous solution with graphite rod as the counter electrode. For comparison, the catalytic performances of the commercial Pt/C (10.0 wt %) (FIG. 30 ), and 1T′-MoS₂ NSs (FIG. 1 ) were also evaluated. As shown in FIG. 4A, the s-Pt/1T′-MoS₂ exhibits a superior activity with an overpotential of only 10 mV to reach a current density of 10 mA cm⁻². Such an overpotential is lower than that of commercial Pt/C (19 mV @ 10 mA cm⁻²). To the best of our knowledge, it is the best among the previously reported Pt-based electrocatalysts (FIG. 39 , Table 3). Moreover, an ultralow overpotential of 30 mV is required for the s-Pt/1T′-MoS₂ to reach a current density of 50 mA cm⁻², which is also much lower than that of commercial Pt/C (71 mV @ 50 mA cm⁻², FIG. 4A). As shown in FIG. 4B, the Tafel slope (28.3 mV dec⁻¹) of the s-Pt/1T′-MoS₂ is similar to that of commercial Pt/C (29.8 mV dec⁻¹). Such a small Tafel slope reveals that the HER catalyzed by the s-Pt/1T′-MoS₂ undergoes a Volmer-Tafel mechanism. In addition, the s-Pt/1T′-MoS₂ exhibits much higher mass activities for the HER than the commercial Pt/C in the whole investigated potential window (FIG. 4C and FIG. 40 , Table 4). In particular, the mass activity of s-Pt/1T′-MoS₂ at 40 mV overpotential was calculated as 6.814 A mg_(Pt) ⁻¹, which is 2.66 times that of the commercial Pt/C (2.557 A mg_(Pt) ⁻¹). By using a CO stripping method, the electrochemically active surface area (ECSA) of s-Pt/1T′-MoS₂ was measured to be 70.3 m² g⁻¹, which is greater than that of commercial Pt/C (41.6 m² g⁻¹). The CO stripping voltammograms are shown in FIG. 31 . At the overpotential of 40 mV, the turnover frequency (TOF) value of s-Pt/1T′-MoS₂ was calculated as 23.1 H₂ s⁻¹, which is higher than that of the commercial Pt/C (14.6 H₂ s⁻¹) (FIG. 41 , Table 5). The aforementioned results demonstrate that the s-Pt/1T′-MoS₂ possesses a significantly enhanced activity for hydrogen generation in comparison with the commercial Pt/C. Furthermore, the stability of s-Pt/1T′-MoS₂ during the HER process was evaluated. As shown in FIG. 4D, the polarization curve shows no obvious degradation after 10,000 cyclic voltammetry cycles, and the structure of the s-Pt/1T′-MoS₂ is well maintained after the durability test (FIG. 32 ). In addition, a negligible variation in the current density of hydrogen evolution is observed after a 30-h test as revealed by the chronoamperometric curve (inset of FIG. 4D).

The voltammogram of commercial Pt/C (FIG. 31A) shows a sharp CO electrooxidation peak which is in accordance with the previous reports. In comparison, the voltammogram of s-Pt/1T′-MoS₂ shows a much broader adsorption peak, which is consistent with the previous reports on single-atomically dispersed catalysts. In FIG. 31B, an optimized Tougaard background was used when integrating the charge under the CO-stripping voltammograms according to the previous report.

After 10,000 cycles of the HER test, the s-Pt/1T′-MoS₂ catalysts were characterized. Obviously, no PtNPs are observed in the TEM and HRTEM images (FIG. 32A-B). The FFT pattern (inset in FIG. 32B) shows the featured pattern of 1T′-MoS₂, in which no diffraction spot can be assigned to the PtNP. The HAADF-STEM image (FIG. 32C) shows that Pt still maintains atomically dispersed on the 1T′-MoS₂ after the HER test. In addition, the EDS mappings (FIG. 32D) show the dispersion of Pt, Mo, and S. All the aforementioned results demonstrate that there is no obvious structural change of the s-Pt/1T′-MoS₂ after the HER stability test, indicating the excellent structural stability of s-Pt/1T′-MoS₂ during the HER process.

All these results confirm the excellent structural stability of s-Pt/1T′-MoS₂ for the HER in acidic solution.

Furthermore, the s-Pt/1T′-MoS₂ and commercial Pt/C were coated on the rotating disk electrode (RDE) and tested in the H₂-saturated 0.5 M H₂SO₄ aqueous solution, respectively, to ensure the H₂/H⁺ equilibrium. The polarization curve tested in H₂-saturated 0.5 M H₂SO₄ aqueous solution shows a negligible difference from that tested in N₂-saturated 0.5 M H₂SO₄ aqueous solution (FIG. 33A). In addition, the Tafel plots (FIG. 33B) indicate that the HER catalyzed by the s-Pt/1T′-MoS₂ in N₂- and H₂-saturated 0.5 M H₂SO₄ aqueous solution both undergo the Volmer-Tafel mechanism. The HER performances of the as-synthesized Pt/1T′-MoS₂ hybrids with different Pt loadings, i.e., s-Pt-4/1T′-MoS₂, s-Pt-6/1T′-MoS₂, PtNPs-12/1T′-MoS₂ and PtNPs-15/1T′-MoS₂, were also tested and compared in FIG. 34 . The HER activity improves as the Pt loading increases from 4.3 wt % to 10.0 wt %, and the s-Pt/1T′-MoS₂ (10.0 wt %) exhibits a much better HER activity than the commercial Pt/C in the Hz-saturated electrolyte. However, further increasing the Pt loading to 12.2 wt % and 15.5 wt % results in the decrease of HER activity. Such results indicate that the high loading amount of s-Pt, i.e., 10.0 wt %, can greatly enhance the HER activity of s-Pt/1T′-MoS₂ hybrids, and the formation of PtNPs at Pt loading of >10.0 wt % would result in a decrease of HER performance.

To further understand the superior HER activity of s-Pt/1T′-MoS₂, DFT calculations were conducted based on the established DFT models of isolated Pt_(ads-S) (FIG. 2E ₁ and FIG. 35A), isolated Pt_(ads-Mo) (FIG. 2F ₁ and FIG. 35B), and Pt_(sub)+Pt_(ads-S)+Pt_(ads-Mo) (FIG. 35C). In order to distinguish from the isolated Pt_(ads-S) and Pt_(ads-Mo), the Pt_(ads-S) and Pt_(ads-Mo) in the Pt_(sub)+Pt_(ads-S)+Pt_(ads-Mo) are denoted as Pt′_(ads-S) and Pt′_(ads-Mo), respectively (FIG. 35A-C). First, the free energy for hydrogen adsorption (ΔG_(H)), which is a well-accepted descriptor for the ability of hydrogen evolution, was calculated to evaluate the HER activity. The optimal value for ΔG_(H) should be close to thermoneutral (˜0 eV), which means that the hydrogen is bound neither too strongly nor too weakly. The calculated ΔG_(H) diagram of Pt_(ads-S), Pt_(ads-Mo), Pt′_(ads-S), and Pt′_(ads-Mo) sites compared to Pt (111) is shown in FIG. 35D. The ΔG_(H) of Pt_(ads-Mo) (−0.07 eV) and Pt′_(ads-Mo) (0.04 eV) are closer to 0 eV as compared to the Pt(111) (−0.11 eV), which could enable the faster hydrogen adsorption and product release on the Pt_(ads-Mo) and Pt′_(ads-Mo) sites, resulting in the superior HER activity of the s-Pt/1T′-MoS₂. Furthermore, the d-band center (ϵ_(d)) was calculated to explain the variation of ΔG_(H) of the s-Pt in s-Pt/1T′-MoS₂ (FIG. 35E). Compared to Pt(111), the shift of ϵ₄ of Pt_(ads-S), Pt_(ads-Mo), Pt′_(ads-S) and Pt′_(ads-Mo) to lower energies indicates that the hydrogen binding strength of the s-Pt in s-Pt/1T′-MoS₂ is weaker than that of Pt(111). With the downshift of ϵ_(d), the ΔG_(H) of the Pt_(ads-Mo) and Pt′_(ads-Mo) close to thermoneutral could contribute to the superior HER activity of the s-Pt/1T′-MoS₂.

The free energies of the hydrogen adsorption (ΔG_(H)) for the isolated Pt_(ads-S), isolated Pt_(ads-Mo), Pt′_(ads-S), and Pt′_(ads-Mo) are calculated to be 0.11, −0.07, 0.20, and 0.04 eV, respectively

(FIG. 35D). Furthermore, the d-band center (ϵ_(d)) is calculated to explain the variation of ΔG_(H) of the s-Pt in s-Pt/1T′-MoS₂. As shown in FIG. 35E, the ϵ_(d) values of isolated Pt_(ads-S) (−2.62 eV) and isolated Pt_(ads-Mo) (−2.58 eV) on 1T′-MoS₂ are much lower than that of Pt(111) (−1.86 eV). Such a shift of ϵ_(d) to the lower energy reveals that the bonding strength between the H atom and the isolated Pt_(ads-S) or Pt_(ads-Mo) is weakened, leading to more positive ΔG_(H) as compared with Pt(111). Specifically, the Pt_(ads-Mo) has a low ΔG_(H) of −0.07 eV, which is even closer to 0 eV than the Pt(111) (−0.11 eV), indicating that the isolated Pt_(ads-Mo) exhibits an HER activity superior to the Pt(111). Moreover, the ϵ_(d) values of Pt′_(ads-S) and Pt′_(ads-Mo) further shift to −2.78 eV and −2.75 eV, respectively, which are lower than that of isolated Pt_(ads-S) and Pt_(ads-Mo), respectively. These results reveal that the hydrogen adsorptions of Pt′_(ads-S) and Pt′_(ads-Mo) are further weakened, resulting in a more positive ΔG_(H) (FIG. 35D). Importantly, the Pt′_(ads-Mo) possesses a nearly thermoneutral ΔG_(H) (0.04 eV), indicating a superior HER activity.

More importantly, the HER activity and stability of the s-Pt/1T′-MoS₂ catalyst at high current densities were also investigated in an H-type cell. The polarization curves (FIG. 4E and FIG. 36 ) indicate that the s-Pt/1T′-MoS₂ can reach high current densities of 1,000, 1,500, and 2,000 mA cm⁻² at low overpotentials of ˜91, 112 and 131 mV, respectively, superior to the commercial Pt/C (at overpotentials of ˜274, 372 and 450 mV, respectively). To the best of our knowledge, this HER performance is the best among the previously reported electrocatalysts in acidic media (FIG. 42 , Table 6). Furthermore, the chronopotentiometric test reveals the outstanding long-term stability of s-Pt/1T′-MoS₂ at a current density of 1,500 mA cm⁻². After 240-h test, the polarization curve exhibits a negligible shift (FIG. 4E) and the overpotential shows no obvious degradation (FIG. 4F). Such high current density and stability demonstrate that the synthesized s-Pt/1T′-MoS₂ could be a promising electrocatalyst towards the large-scale hydrogen production through water splitting.

In summary, the controlled preparation of 1T′-MoS₂ nanosheets (NSs) with high phase purity and illustrated the crystal phase effect of MoS₂ NSs on the templated growth of Pt is described herein. Specifically, the single-atomically dispersed Pt (s-Pt) with high Pt loading of ˜10.0 wt % can be formed on the 1T′-MoS₂ NSs. Importantly, the obtained s-Pt/1T′-MoS₂ exhibits superior electrocatalytic HER performance, outperforming the commercial Pt/C and the previously reported Pt-based electrocatalysts. Impressively, s-Pt/1T′-MoS₂ can reach a high current density of 1,000, 1,500 and 2,000 mA cm⁻² at low overpotentials of ˜91, 112 and 131 mV, respectively, which are the best among the reported electrocatalysts. Moreover, the s-Pt/1T′-MoS₂ can work continuously and steadily at a high current density of 1,500 mA cm⁻² for 240 h without any obvious degradation, showing great potential for the practical application. This work demonstrates that the crystal phase of 2D nanomaterial is an important and effective factor to control the templated growth of materials with different structure. It also paves the way for the rational design and construction of hybrid structures with unique properties and superior performance towards various applications.

Examples Materials Synthesis Chemicals.

Potassium molybdate (K₂MoO₄, 98%), sulfur powder (S, 99.5%), isopropanol (IPA, 99.8%), acetonitrile (99.8%), potassium tetrachloroplatinate (II) (K₂PtCl₄, 99.99% trace metals basis), gold(III) chloride hydrate (˜50% Au basis), silver nitrite (99.98% trace metals basis), Nickel(II) nitrate (99.999% trace metals basis), Hydrogen hexachloroiridate(IV) hydrate (99.9% trace metals basis), Oleylamine (OAm, 70%), 1-octadecene (ODE, 90%), Hexane (ReagentPlus®, ≥99%), poly(vinylidene fluoride) (PVDF), tetraheptylammonium bromide, and platinum on carbon (10 wt. % loading, matrix activated carbon support) were purchased from Sigma-Aldrich. Tin (II) chloride (SnCl₂, 99%), bismuth (III) chloride (BiCb, 99.9%), copper (II) chloride (CuCl₂, 99.9%) and n-butyllithium (n-BuLi, 1.6 M solution in hexane) were purchased from Alfa Aesar. Purified argon (Ar, 99.9%) and hydrogen (20% H₂/80% Ar) were purchased from Leeden National Oxygen Ltd. (Singapore). Copper foils were purchased from ACME Research Support Pte Ltd (Singapore). Ethanol (99.9%) and acetone (Tech Grade) were purchased from Merck (Germany). N,N-dimethylformamide (DMF) was purchased from Fisher Scientific. All chemicals and materials were used as received without any further purification. The Milli-Q water (resistivity of 18.2 MΩ·cm, Milli-Q System, Millipore, Billerica, MA, USA) was used in our experiment.

Synthesis of K_(x)MoS₂ Crystals

K₂MoO₄ (500 mg) and S powder (500 mg) were mixed and ground, the mixture was placed in a quartz tube and annealed in a tube furnace at 450° C. for 1.5 h under a gas flow of H₂ (10 sccm) and Ar (190 sccm). After cooling down to room temperature, the product was taken out and then mixed with S powder (500 mg). The obtained mixture was placed in a quartz tube and annealed again in the tube furnace at 450° C. for 1.5 h under an atmosphere of H₂ (10 sccm) and Ar (190 sccm). Subsequently, the reaction zone was heated to 850° C. at a rate of 30° C. min⁻¹ under an atmosphere of H₂ (40 sccm) and Ar (160 sccm), and then maintained at 850° C. for 10 h. After cooling down to room temperature, the obtained powder was collected and washed with Milli-Q water until the pH value of the suspension reached 7-8. The obtained powder was then stored in Milli-Q water for 24 h. After washing with Milli-Q water again and drying at room temperature under vacuum, the K_(x)MoS₂ crystals were obtained and collected for the further usage.

K_(x)MoSe₂ crystals were synthesized by the similar process as K_(x)MoS₂ crystals, while Se powder was used instead of S powder. K_(x)MoSSe crystals were synthesized by the similar process as K_(x)MoS₂ crystals, while Se/S mixed powder (molar ratio 1:1) was used instead of S powder. K_(x)WS₂ were synthesized by the similar process as K_(x)MoS₂ crystals, while K₂WO₄ powder was used instead of K₂MoO₄ powder.

Preparation of 1T′-MoS₂ Nanosheets (NSs) by Electrochemical Intercalation.

The electrochemical intercalation process was conducted in a two-electrode electrochemical cell. After the K_(x)MoS₂ crystals and PVDF as a binder were mixed in DMF in a mass ratio of K_(x)MoS₂ crystals, PVDF, and DMF of 8:1:80, the mixture was uniformly coated on a copper foil and dried under vacuum, which was used as the cathode. A graphite rod was used as the anode. Tetraheptylammonium bromide, which was dissolved in acetonitrile with a concentration of 5 mg/ml, served as electrolyte. The intercalation process was performed for 1 h at an applied voltage of 8 V. The intercalated sample was then transferred into a centrifuge tube followed by sonication in 5 ml of DMF for less than 5 s. The dispersion was centrifuged at 6,000 r.p.m. for 10 min. The obtained precipitate was re-dispersed in 5 ml of Milli-Q water. The final product was collected by centrifugation at 6,000 r.p.m. for 10 min and re-dispersed in Milli-Q water for the further usage. 1T′-MoSe₂, 1T′-MoSSe, and 1T′-WS₂ NSs can also be prepared by using the similar process while the K_(x)MoSe₂, K_(x)MoSSe, K_(x)WS₂ crystals are used instead of the K_(x)MoS₂ crystals.

Synthesis of Different Pt Structures on 1T′-MoSNSs.

In a typical experiment to prepare the single-atomically dispersed Pt on 1T′-MoS₂ (s-Pt/1T′-MoS₂), 120 μl aqueous solution of 0.05 M K₂PtCl₄ were injected into 10 ml of 1T′-MoS₂ water-ethanol (v/v=9:1) solution (0.10 mg/ml, determined by inductively coupled plasma-optical emission spectrometry (ICP-OES)) in a 15-ml glass vial. The obtained mixture was then irradiated under a 150 W halogen lamp at 10% of its full intensity for 14 h under ambient conditions. After the photoreduction reaction, the resulting solution was centrifuged at 6,000 r.p.m. for 15 min. The precipitates were washed with IPA and collected for the further usage. Based on the ICP-OES result, the Pt loading in the prepared s-Pt/1T′-MoS₂ was determined to be 10.0 wt % (FIG. 37 , Table 1). The aforementioned synthetic method was also used to prepare different Pt structures on the 1T′-MoS₂ NSs with the Pt loadings of 4.3 wt % (s-Pt-4/1T′-MoS₂), 6.4 wt % (s-Pt-6/1T′-MoS₂), 12.2 wt % (PtNPs-12/1T′-MoS₂), and 15.5 wt % (PtNPs-15/1T′-MoS₂) (as determined by the ICP-OES, FIG. 37 , Table 1), when 120 μl aqueous solution of 0.05 M K₂PtCl₄ was changed to 80, 100, 200, and 300 μl, respectively.

Synthesis of Other Single-Atomically Dispersed Metal/TMD Hybrids in an Aqueous Phase.

The aforementioned synthetic method was also used to prepare other metal/1T′-MoS₂ hybrid structures. For example, by using the similar process, single-atomically dispersed Au (s-Au) can be grown on the 1T′-MoS₂, while HAuCl₄ (10 μl aqueous solution of 0.05 M HAuCl₄) is used as the Au precursor instead of K₂PtCl₄. The aforementioned synthetic method was also used to prepare other metal/TMD hybrid structures. For example, single-atomically dispersed Pt (K₂PtCl₄) can also be grown on the 1T′-WS₂ to form the s-Pt/1T′-WS₂, while the 1T′-WS₂ NSs are used as templates instead of the 1T′-MoS₂. In addition, by simply changing the precursor to NiNO₃, H₂IrCl₆ or AgNO₃, s-Ni/1T′-WS₂, s-Ir/1T′-WS₂ or s-Ag/1T′-WS₂ can also be prepared respectively.

Synthesis of Other Single-Atomically Dispersed Metal/TMD Hybrids in an Oil Phase.

This method can also be extended to oil phase system. For example, single-atomically dispersed Sn can be grown on the 1T′-MoS₂. The as-exfoliated 1T′-MoS₂ was dispersed in 10 mL of oleylamine (OAm, 100 μg/ml) in a three-neck round-bottom flask. The flask was degassed under vacuum for 1 h. 10 mg of SnCl₂ was dissolved into 10 mL of OAm by sonication. Then, 10 μL of this mixed solution were injected into the three-neck round-bottom flask under argon flow. After the flask was heated to 40° C. (˜10° C./min from room temperature) under magnetic stirring and flowing argon, 0.1 mL of n-butyllithium was injected and then kept at 40° C. for 20 min. After natural cooling down to room temperature, 10 mL of hexane and 20 mL of ethanol were added into the solution to precipitate the s-Sn/1T′-MoS₂ by centrifugation at 4000 rpm for 5 min. The precipitate was further purified three times using a hexane/ethanol mixture with a volumetric ratio of 1:1, and finally dissolved into hexane for further characterizations. The aforementioned synthetic method was also used to prepare other metal/TMD hybrids structures such as s-Bi/1T′-MoS₂ s-Sn-Bi/1T′-MoS₂, and s-Sn—Bi—Cu/1T′-MoS₂ by simply changing the precursor to SnCl₂, SnCl₂+BiCl₃, and SnCl₂+BiCl₃+CuCl₃.

Characterization of Materials

Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images, selected area electron diffraction (SAED) patterns, scanning transmission electron microscopy (STEM) and the corresponding energy-dispersive X-ray spectroscopy (EDS) data were recorded on JEOL JEM-2100F (JEOL, Tokyo, Japan) at an acceleration voltage of 200 kV. Aberration-corrected high-angle annular dark-field STEM (HAADF-STEM) images were obtained on a JEOL ARM-200F (JEOL, Tokyo, Japan) operated at 200 kV with cold field emission gun and double hexapole spherical aberration correctors (CEOS GmbH, Heidelberg, Germany). The STEM image simulations were conducted with QSTEM, a STEM image simulation software. Scanning electron microscope (SEM) images and the corresponding EDS spectra were recorded on JEOL JSM-7600F (JEOL, Tokyo, Japan). Optical microscopy images were taken on a Nikon Eclipse LV100D microscope. Atomic force microscope (AFM, Cypher, Asylum Research, USA) was used to characterize the thickness of 1T′-MoS₂ NSs in tapping mode in air. Ultraviolet-visible (UV-Vis) spectra were recorded on a UV-2700 (Shimadzu, Tokyo, Japan) with QS-grade quartz cuvettes (111-QS, Hellma Analytics) at room temperature. X-ray photoelectron spectroscopy (XPS) measurements were conducted on the ESCALAB 250Xi (Thermo Fisher Scientific, USA) instrument. Raman spectra were recorded by the WITec system (Germany) with a wavelength of 532 nm and power of <0.1 mW to avoid the phase transformation of MoS₂ during the measurement. The Raman band of a Si wafer at 520 cm⁻¹ was used as the reference to calibrate the spectrometer. X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) measurements of the platinum L₃ edge were carried out using the X-ray absorption fine structure for catalysis beamline in Singapore Synchrotron Light Source. Processing and analysis of data were carried out on Athena and Artemis (version 0.9.26). Simulation of XANES was implemented in the finite difference method near edge structure (FDMNES) code, in which the Schrodinger equation was solved by the finite difference method (FDM) within the local density approximation. ICP-OES was performed on a Dual-view Optima 5300 DV ICP-OES system (USA).

Electrochemical Measurements

Hydrogen evolution reaction (HER) measurements were conducted in a conventional three-electrode system using an Autolab electrochemical workstation (PGSTAT12) under ambient conditions. The graphite rod and Ag/AgCl (3 M KCl) were used as counter electrode and as reference electrode, respectively. The Ag/AgCl electrode was calibrated with respect to a reversible hydrogen electrode (RHE). The working electrode was prepared by drop-casting the s-Pt/1T′-MoS₂ dispersion in IPA onto a GCE (3 mm in diameter) with the Pt loading amount of 0.01 mg cm⁻² measured by ICP-OES. After the catalyst-coated GCE was dried at room temperature, 2 μl of Nafion ethanolic solution (0.1 wt %) were dropped on its surface to protect the catalyst. Using the same procedure, 1T′-MoS₂ NSs, and Pt/C were also loaded on GCE. The weights of 1T′-MoS₂ NS s, and s-Pt/1T′-MoS₂ were kept the same (0.1 mg cm⁻²). The mass loadings of Pt were kept the same (0.01 mg cm⁻²) for s-Pt/1T′-MoS₂, and Pt/C. After drying, the electrodes were used for the electrochemical measurements. The HER was conducted in 0.5 M H₂SO₄ aqueous solution (purged by pure N₂). Linear sweep voltammetry (LSV) curves were measured in a N₂-saturated 0.5 M H₂SO₄ aqueous solution at a scan rate of 5 mV s⁻¹. The durability tests were performed by applying the cyclic potential sweeps between 0.1 V and −0.1 V (vs. RHE) at a scan rate of 100 mV s⁻¹ for 10,000 cycles. The chronoamperometric test was conducted in a N₂-saturated 0.5 M H₂SO₄ aqueous solution for 30 h. Electrochemical impedance spectroscopy (EIS) was recorded over the frequency range from 100 kHz to 0.1 Hz with an amplitude of applied voltage of 10 mV. All the LSV curves were iR-corrected on basis of the EIS data. Current densities were normalized by the geometric area of the electrode.

Turnover efficiency (TOF) values of catalysts were calculated from the number of active sites which was obtained by using the CO stripping methods. The CO adsorption was conducted in 0.5 M H₂SO₄ aqueous solution. While maintaining the potential of working electrode at 0.1 V (vs. RHE), CO was bubbled into 0.5 M H₂SO₄ aqueous solution for 20 min to ensure the saturated adsorption of CO on the surface of the catalyst. The electrolyte was then saturated with N₂ by bubbling N₂ for 15 min to remove the dissolved CO in the electrolyte. CO stripping voltammograms were then recorded. The number of active sites (n) was calculated on basis of the CO stripping charge (Q_(CO)) using the following equation:

n=Q _(CO)/(2Fm)  (1)

where F is the Faraday constant (96,485 C mol⁻¹), and m is the metal (Pt) mass loading (here, the Pt mass loadings of s-Pt/1T′-MoS₂ and commercial Pt/C are kept as 0.7×10⁻⁶ g, measured by ICP-OES). The turnover frequency (TOF, H₂ s⁻¹) can be calculated by using the following equation:

TOF=I/(2Fnm)  (2)

where I is the current (A) during the LSV measurement. The factor, 2, is the number of electron transferred, because two electrons are required to form one H₂ molecule. Assuming a value of 420 μC cm⁻² for a saturated CO monolayer formation on active metal sites, the electrochemically active surface area (ECSA) can be calculated using the following equation:

ECSA=Q _(CO)/(m×420 μC cm⁻²)  (3)

The s-Pt/1T′-MoS₂ was also tested in the H₂-saturated 0.5 M H₂SO₄ electrolyte on the rotating disk electrode (RDE) (3 mm in diameter, Pine research instrument, USA) with the Pt loading amount of 0.01 mg cm⁻² (measured by ICP-OES). After drop-casting the s-Pt/1T′-MoS₂ catalyst on the RDE and drying under ambient condition, 2 μl of Nafion ethanolic solution (0.1 wt %) were dropped on the surface to protect the catalyst. By using the same procedure, s-Pt-4/1T′-MoS₂, s-Pt-6/1T′-MoS₂, PtNPs-12/1T′-MoS₂, PtNPs-15/1T′-MoS₂ and commercial Pt/C samples were also coated on RDE with the same Pt loading amount (0.01 mg cm⁻², measured by ICP-OES). The HER tests were conducted in a conventional three-electrode system with graphite rod as counter electrode and Ag/AgCl (3 M KCl) as reference electrode. All the tests were conducted at a rotating rate of 1,600 r.p.m. at a scan rate of 5 mV s⁻¹. Electrochemical impedance spectroscopy (EIS) was recorded over the frequency range from 100 kHz to 0.1 Hz with an amplitude of applied voltage of 10 mV. All the LSV curves were iR-corrected on basis of the EIS data. Current densities were normalized by the geometric area of the electrode.

The HER activity and stability of s-Pt/1T′-MoS₂ and Pt/C at high current densities were tested in an H-type cell separated by an ion exchange membrane (Nafion 117). The working electrode was prepared by drop-casting the s-Pt/1T′-MoS₂ or Pt/C dispersion onto a carbon fiber paper, in which the Pt loading density was kept at 0.0175 mg cm⁻² measured by ICP-OES. After the catalyst-modified carbon fiber paper was dried at room temperature, 5.7 μl of Nafion ethanolic solution (0.1 wt %) were dropped on its surface to protect the catalyst. Pt mesh and Ag/AgCl (3 M KCl) were used as the counter electrode and reference electrode, respectively. The LSV curves were measured in a N₂-saturated 0.5 M H₂SO₄ aqueous solution at a scan rate of 5 mV s⁻¹, and then iR-corrected on basis of the EIS data. The chronopotentiometric test at 1,500 mA cm⁻² was conducted in a N₂-saturated 0.5 M H₂SO₄ aqueous solution for 240 h. After the long-term stability test, the LSV curve of s-Pt/1T′-MoS₂ was measured again.

Density Functional Theory (DFT) Calculations

DFT calculations were performed by using the projector augmented wave (PAW) method¹¹ as implemented in the Vienna ab initio simulation package (VASP 5.4. The generalized gradient approximation in the revised-Perdew-Burke-Ernzerhof (RPBE) form was used, and a cutoff energy of 400 eV for plane-wave basis set was adopted. The convergence thresholds were 10⁻⁵ eV and 0.01 eV/A for energy and force, respectively. A vacuum space of at least 15 Å was used, so that the interaction between two periodic units can be neglected. Supercells consisting of 2×3×1 of the 1T′-MoS₂ monolayer were used to simulate the 1T′-MoS₂ NS s, and the Brillouin zones were sampled by a 5×5×1 Monkhorst-Pack k-point grid.

The free energy for hydrogen adsorption (ΔG_(H)) was adopted to theoretically evaluate the catalytic performance for HER, which was calculated using the equation,

ΔG _(H) =ΔE+ΔE _(ZPE) −TΔS  (4)

where the ΔE is the adsorption energy of hydrogen, ΔE_(ZPE) is the correction of zero-point energy, ΔS represents the difference in entropies between the adsorbed state and the corresponding free-standing state, and T is the absolute temperature (300 K). 

What is claimed is:
 1. A single-atomically dispersed metal/two-dimensional transition-metal dichalcogenide nanosheet hybrid (TMD NS hybrid) comprising a plurality of single-atomically dispersed metal atoms disposed on at least one surface of a transition-metal dichalcogenide nanosheet (TMD NS), wherein the transition-metal dichalcogenide nanosheet is uniformly crystalline.
 2. The TMD NS hybrid of claim 1, wherein each of the plurality of single-atomically dispersed metal atoms is ruthenium, rhodium, palladium, silver, osmium, iridium, platinum, gold, iron, cobalt, nickel, copper, zinc, cadmium, indium, tin, antimony, lead, bismuth, or other metal atoms.
 3. The TMD NS hybrid of claim 1, wherein each of the plurality of single-atomically dispersed metal atoms is platinum, gold, nickel, iridium, silver, tin, bismuth, or copper.
 4. The TMD NS hybrid of claim 1, wherein the TMD NS comprises MoS₂, MoSe₂, MoTe₂, WS₂, WSe₂, MoTe₂, WTe₂, TiS₂, TiSe₂, TaS₂, TaSe₂, VS₂, VSe₂, NbS₂, NbSe₂, ReS₂, ReSe₂, MoS_(2(1-A))Se_(2A), or WS_(2(1-A))Se_(2A), wherein A is 0-1.
 5. The TMD NS hybrid of claim 4, wherein the crystal phase of the transition-metal dichalcogenide is 1T′ phase.
 6. The TMD NS hybrid of claim 1, wherein the TMD NS comprises 1T′-MoS₂, 1T′-MoSe₂, 1T′-MoSSe, or 1T′-WS₂.
 7. The TMD NS hybrid of claim 1, wherein the TMD NS comprises 1T′-MoS₂.
 8. The TMD NS hybrid of claim 1, wherein each of the plurality of single-atomically dispersed metal atoms is platinum, gold, nickel, iridium, silver, tin, bismuth, or copper and the TMD NS comprises 1T′-MoS₂, 1T′-MoSe₂, 1T′-MoSSe, or 1T′-WS₂.
 9. The TMD NS hybrid of claim 1, wherein each of the plurality of single-atomically dispersed metal atoms is platinum and the TMD NS comprises 1T′-MoS₂.
 10. The TMD NS hybrid of claim 1, wherein the plurality of single-atomically dispersed metal atoms is present in the TMD NS hybrid at a weight percentage of 12.2 wt % or less.
 11. The TMD NS hybrid of claim 1, wherein the plurality of single-atomically dispersed metal atoms are present in the TMD NS hybrid at a weight percentage of 10.0 wt % or less.
 12. The TMD NS hybrid of claim 1, wherein each of the plurality of single-atomically dispersed metal atoms is gold or platinum; the TMD NS comprises 1T′-MoS₂; and the plurality of single-atomically dispersed metal atoms are present in the TMD NS hybrid at a weight percentage of 10.0 wt % or less.
 13. A method of preparing the TMD NS hybrid of claim 1, the method comprising: contacting a TMD NS with a plurality of single-atomically dispersed metal atom precursors in the presence of a reducing agent thereby forming the TMD NS hybrid, TMD NS is uniformly crystalline.
 14. The method of claim 13, wherein each of the plurality of single-atomically dispersed metal atom precursors are metal salts comprising at least one metal atom.
 15. The method of claim 13, wherein the plurality of single-atomically dispersed metal atom precursors is selected from the group consisting of M₂PtX₄, M₂PtX₆, M₂IrX₆, MAuX₄, SnY₃, BiY₃, CuY₂, AgY, NiY, wherein X is halide and Y is nitrate, cyanide, formate, acetate, or acetylacetonate; and M is hydrogen, lithium, sodium, potassium, or cesium.
 16. The method of claim 13, wherein the reducing agent is ascorbic acid, sodium citrate, metal hydride, H₂, hydrazine, alcohol, organolithium, electrochemical reduction, or photoreduction optionally in the presence of an additional reducing agent.
 17. The method of claim 13, wherein the plurality of single-atomically dispersed metal atom precursors is K₂PtCl₄, H₂IrCl₆, HAuCl₄, SnCl₃, BiCl₃, CuCl₂, AgNO₃, or NiNO₃, and the reducing agent is photoreduction in the presence of an alcohol or chemical reduction by using n-butyllithium as reducing agent.
 18. The method of claim 17, wherein the plurality of single-atomically dispersed metal atoms are present in the TMD NS hybrid at a weight percentage of 10.0 wt % or less.
 19. An electrode comprising a base electrode and the TMD NS hybrid of claim 1, wherein the base electrode is a planar electrode, including the glassy carbon electrode, a graphite electrode, an indium tin oxide (ITO) electrode, a fluorine doped tin oxide (FTO) electrode, a gas diffusion electrode (GDE), carbon paper electrode, carbon fiber electrode, polycarbonate track etch (PCTE)-based electrode, or titanium-based electrode.
 20. An electrochemical cell comprising: a cathode comprising the TMD NS hybrid of claim 1; an anode; and an electrolyte.
 21. A method of producing hydrogen gas, the method comprising reducing a proton source at the cathode of the electrochemical cell of claim 20 thereby producing hydrogen gas, wherein the proton source is water optionally comprising an acid. 